chick rearing and cooperative breeding -- chick rearing depends a lot on egg size and incubation...

Chick Rearing and Cooperative Breeding

-- chick rearing depends a lot on egg size and incubationperiod

-- larger eggs, longer incubation, chick more developed athatch

-- in general, larger birds produce smaller clutches withlarge eggs; small birds the reverse

Some exceptions do occur:

-- passerines lay proportionally larger eggs perbody mass than other birds

-- kiwi has largest egg per body mass at 25%

-- takes female 33 days to produce egg, 75-80to incubate

-- chick hatches precocial, feeds on its own

precocial eggs will have 40-50% yolk by weight, 2-3X larger thanaltricial eggs of bird with same body mass

Factors that control egg size include:

1. Food availability

-- a stable food source selects for larger eggsand precocial young

-- food variable or scarce, small eggs and altricialyoung

Royal Tern cormorant

2. Predation

-- altricial young are helpless, take longer to raise

-- constant feeding trips can draw attention of predators

-- longer time at one spot, better chance of being found

In tropics, lots of predators but only 15% of birds have precocial young while 35% are altricial

-- high risk in losing nest to predation selects againstlarge investment in large eggs

-- better to have small eggs, easily replaced if lost topredation, than large eggs with large energetic loss

-- in temperate regions, more precocial eggs as food is moreseasonal, less predation, and cooler temperatures

3. Clutch size

-- a bird can produce many small eggs but only 1-2 large

-- cannot produce many large eggs

-- in general, birds should be selected to produce the maximumnumber of eggs possible for prevailing conditions

Food Limitation Hypothesis

-- first proposed by David Lack (1911-1973)

-- well known British ecologist, wrote many books includingEcological Isolation in BirdsPopulation Studies in Birds,Darwin’s Finches

Tests of Lack’s hypothesis had mixed results:

-- some birds fit the model, others did not

-- some species could raise more young, but don’t

-- e.g., gulls with eggs added to nest raise all young

-- but…pay the price in future survival

There are natural ways for gulls and other birds to avoidexpending too much energy for breeding

Third Chick Disadvantage

Brandt’s Cormorants can lay 5-6 eggs, but vary amount each year based on winds, upwelling

-- use proximate factors to adjust timing of nestconstruction and clutch size

-- will abandon eggs or young if food supply drops

-- selects for limiting reproductive effort each year, highsurvivorship of parents, and high lifetime reproductivesuccess

Siblicide also may occur as a means to reduce clutch size

-- can be obligate or facultative

-- behavior stimulated by hunger

-- obligate in egrets, south polar skua

Preconditions for siblicide behavior to evolve:

1. clutches of two or more young

2. limited food supplies, occasional or consistent

3. small amounts of food fed directly to chicks byadults, setting up fights between chicks

4. weapons are needed (hooked bill)

5. small nests and territories, no escape

6. hatching asynchrony so that first chick is larger

Chick Growth

-- typical pattern is that growth is slow at first, then goesrapidly until close to adult weight

-- slow start especially for altricial chicks, can’t thermoregulateyet and need brooding by parent

-- once physiology is developed, chick growth rate increases

-- many birds fledge, though, at weights above or belowaverage adult weight

Aerial insectivores such as swallows may fledge above adult weight

-- more muscle mass in wings while learning how to forage-- tissues then regress to normal size and weight

Seabirds may have more water in tissues to help them grow at first

Most chicks fledge below adult weight and continue growingwhile feeding on their own

-- ground-nesting birds may have more growth in legs thanwings at first, helps them leave nest and forage

Some seabirds may have large feet in chicks to help them swim

-- murrelets leave nest after 1-2 days to forage at sea

-- feet unusually large for body size, then body catches up

-- common murres take chicks to sea when half grown

-- male takes chick after they jump from cliffs

Chick growth rates and thermoregulation can be facilitatedwhen they form creches

-- chicks may gather at one nest in a colony to huddle togetherwhile parents out foraging

-- run back to own nest when parents return (cormorants)

-- also get creches outside of nests in common areas

-- can be for huddling and thermoregulation (penguins) or forgreater protection from predators (terns)

Brood sizes of 2 or more chicks can be advantageous toboth chicks and adults for growth and energy savings

-- titmice in England required less food per chick with larger broods

-- with three chicks, feed 1.6 to 1.9 g per day

-- with 13, ate only 0.6 to 0.8 g per day

Heat exchange among chicks in larger brood kept need for more food to a minimum

Cooperative Breeding in Birds

-- when more than just parents participate in feedingand raising young

-- ‘helpers’ at the nest, usually young from previousbreeding

-- once thought rare in birds, now known in > 220 species

How does helping behavior evolve?

-- appears altruistic—why help when the offspring are not yours?

-- obvious advantage to the breeding pair withincreased care and protection of young

-- advantage to helpers varies in each system

The causes for the behavior may include:

1. non-adaptive hormonal stimulation-- birds want to breed, but can’t-- end up helping to act out behavior stimulated

by hormones-- breeders tolerate the helper

2. Inclusive fitness-- helpers are related in part to young in nest (same

parents) so share genes

3. Risk of dispersal-- helpers are often young of previous year-- delay leaving natal territory due to risks of

of dispersal

4. Experience -- helpers learn how to care for and raise young -- this experience will help them become more

successful breeders later, especially first time

5. Access to mates-- some helpers may form pair bond to one of the

breeding pair if the pair dissolves or one matedies

-- this would explain helpers not related to breedingpair

Several long-term studies now address some of thesepoints for evolution of helping behavior

Acorn Woodpeckers

-- studied by Walt Koenig at Hastings Reserve, CAfor over 25 yrs

-- birds form ‘clans’ of 10-15 related birds

-- 1-3 breeding females per year

-- males compete for mating with females, 1-4may mate

-- all eggs laid in one nest cavity

All clan members help incubate eggs, feed young

Young helpers stay in clan to avoid dispersal

Breeding pair, when dies, may be replaced by an unrelated pair outside the clan

Large clans with many helpers fledge more chicks than small clans

Helping behavior especially important in bad food years

Pygmy Nuthatch

-- have social groups with one breeding pairand 1-3 helpers, often ‘threesomes’

-- helpers usually male offspring from previous year

-- helpers help feed young, mob predators

-- use tree cavities for nesting and for winterroosts

-- nests with helpers have less predation

-- helpers have access to winter social groups, mates

Study in Arizona in 1980s showed helpers did not make much difference in breeding success

-- only one out of four years of study showed a benefit to breeding pair with helpers vs. those without

-- real advantage to helper with access to winterroost cavities, increases survivorship in winter

-- record count for winter roost: 167 birds!

Florida Scrub-jay

-- studied for >25 yrs in Florida by Glen Woolfenden

-- up to 55% of nests have helpers, usually male offspring from previous breeding

-- groups have large territories, up to 9 ha, for one monogamous pair plus 1-6 prebreeding helpers

Scrub/oak/palmetto habitat

-- poor habitat, so large territory and helping needed to be successful and survive

-- this habitat unlike richer oak woodlands in western U.S.where jays do not have helpers

Most Florida scrub-jays delay breeding until2-3 yrs old

-- helpers are 1-2 yr old prebreeders

-- by age 3, most are breeding on own territories:98% female and 77% male of cohort arebreeding by this age

-- most helpers are males, may be able to obtainterritory if breeding male dies

Helpers act as ‘sentinels’ to watch for predatorswhile other group members feed

-- will give alarm calls to warn others

Do helpers gain from their experience?

study showed that:

1-yr helper 2+ yr helperSurvivorship 0.86 0.72RS 2.03 1.74

These results indicate no advantage for survivorshipif helping two or more years

No increase in RS for helping longer either

Woolfenden concluded that long-term helpers are poorquality males, less fit than those that help one year andthen get territory.

Less fit helpers begin wandering after age 2-3:

Wandering Inept Male Prebreeders